Well I thought that too. I might want to play the game again.
There was also a tidbit in end of the Terran Campaign trailer, that has Zeratul saying: "The Xel'Naga have returned..." In order to get the bonus mission, you have to destroy Zeratul's base in one of the last Zerg missions in less than 20 minutes.
oh....did they include Movable Xel'naga enemies or something? because i never got that bonus mission =( i only the ones where the protoss (i can't remember who) were shocked at the experiments inside the stasis chambers at the frozen lands.